This invention relates generally to a multiple-diode raster output scanning system. The system directs high intensity imaging beams toward a reflective scanning element, causing the beams to scan across a movable photoresponsive member and record latent electrostatic images thereon. More particularly, the invention relates to the use of a multi-channel integrated circuit for control of laser beams and, in turn, the exposure of the photoresponsive surface.
In recent years, laser printers have been increasingly used to produce output copies from input video data representing original image information. Laser printers typically uses a Raster Output Scanner (ROS) to expose charged portions of the photoresponsive member to record an electrostatic latent image thereon. Generally, a ROS has a laser and associated optical components for generating a collimated beam of monochromatic radiation. The laser beam is modulated in conformance with the image information (e.g., video data). The modulated beam is reflected through a lens onto a scanning element such as a rotating polygon having mirrored facets.
The light beam is reflected from a facet of the polygon and thereafter focused to a xe2x80x9cspotxe2x80x9d on the photosensitive member. The rotation of the polygon causes the spot to scan across the photoresponsive member in a fast scan (i.e., line scan) direction. Meanwhile, the photoresponsive member is advanced relatively more slowly than the rate of the fast scan in a slow scan (i.e., process) direction that is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster (or scan line) scanning pattern. The light beam is modulated in accordance with an input image data stream at a rate such that individual picture elements (xe2x80x9cpixelsxe2x80x9d) of the image represented by the data stream are exposed on the photosensitive medium to form a latent electrostatic image. The latent image is then developed and transferred to an appropriate image receiving medium such as paper.
When using a multiple laser diode for simultaneous imaging with at least two emitted laser beams, it is very important to balance the power of the beams and modulation of the beams to provide uniform exposure of the photoresponsive member. A lack of uniformity can create image quality problems. The problems are exacerbated by the presence of both thermal crosstalk between laser diodes placed within a common package and timing differences in the electronics used to modulate the laser beams. Differences in timing between the channels used to modulate each of the beams (e.g., propagation delay), results in image defects in the developed electrostatic image (e.g., xe2x80x9ccity blocksxe2x80x9d defects).
It is desirable to provide a multiple diode modulation control system that overcomes the timing differential difficulties observed in multiple-channel ROS systems. Two characteristics that are critical to a multi-channel ROS modulation control are channel-to-channel differential propagation delay and channel-to-channel differential linearity. Channel-to-channel differential propagation delay is measured from the rising edge of the pixel clock and is the difference between the signal level on each channel when the pulse-width and position-modulated (PWPM) pulse occurs. Channel-to-channel differential linearity is the difference between the measure of pulse width and position for the same video data input code applied to each channel, as measured over the range of video input codes.
It is therefore an object of the present invention to reduce channel-to-channel differential propagation delay and channel-to-channel differential linearity in the ROS modulation control so as to eliminate image quality defects that arise from significant timing differentials. Another object of the present invention is to reliably produce matched channel devices so as to eliminate the need to pre-sort or compensate for differential propagation delay or differential linearity in paired single-channel devices. Additionally, placing multiple PWPM channels on a common integrated circuit results in both a cost savings for the integrated circuit, and a reduction in printed wiring board space. Other advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.
This invention is a ROS system that uses a multi-channel modulation control device, implemented with a common integrated circuit, to control the power from a multiple (preferably dual or quad) laser diode. The present invention has particular application to digital image printing and reprographic systems. The invention is believed to result in both a cost savings for such systems and improved image quality relative to conventional dual diode ROSs.
In accordance with the present invention, there is provided a raster imaging system for exposing a photoresponsive surface moving relative to the raster imaging system in a process direction, including: a multi-beam laser diode scanner forming a plurality of rasters across said surface, in a direction transverse to the process direction, by reflecting and modulating a plurality of beams so as to direct the beams to the photoresponsive surface; a controller to control the power from the multi-beam laser diode, said controller including, means to control, individually, the exposure level from each of the plurality of laser diodes by controlling the laser diode current; and a multi-channel beam modulation controller, providing video signals to each of the multi-beam lasers, wherein the multi-channel beam modulation controller is implemented in a single integrated circuit package.
In accordance with another aspect of the present invention, there is provided a printing system for forming multiple image exposure frames on a photoresponsive surface moving in a process direction and subsequently developing an transferring the developed images to a printable medium, including: a multiple beam raster output scanner forming a plurality of scan lines in a transverse direction across a width of the member by reflecting modulated beams from a plurality of facets of a rotating polygon, wherein the raster output scanner employs a multi-beam laser diode; means for detecting the beginning of a scan line as a modulated beam traverses a start-of-scan position and providing a start of scan (SOS) signal representing the detection, wherein the SOS signal is subsequently used to synchronize the rotation of the rotating polygon with movement of the photoresponsive surface; a controller to control the power from the multi-beam laser diode, said controller including, means to control, individually, the exposure level from each of the plurality of laser diodes by controlling the laser diode current; and a multi-channel beam modulation controller, providing video signals to each of the multi-beam lasers, wherein the multi-channel beam modulation controller is implemented in a single integrated circuit package
One aspect of the invention is based on the ability to place multiple channel modulation control (PWPM or high-addressability) on a single integrated circuit device. The system described herein is advantageous because it is efficient and inexpensive compared to other approaches, and eliminates a substantial source of image quality defects in multi-channel, multi-beam raster output scanning systems.